Unlike Earth's magma, which tends to cluster in pockets around
the edges of tectonic plates, Io's magma is found in a global
reservoir at least 30 miles (48 kilometers) deep, the study
suggests. This huge reserve of subsurface molten rock helps
explain why Io is the
most volcanically active object in the solar system, spewing
out 100 times more lava than all of Earth's volcanoes combined.

"Now we know where all of that lava is coming from," said study
lead author Krishan Khurana, a geophysicist at UCLA. [ Top 10
Extreme Planet Facts ]

The most volcanic body in the solar system

Io, the third-largest of Jupiter's many satellites, is just a
touch bigger than Earth's moon. It orbits about as close to
Jupiter, on average, as our moon does to Earth.

This proximity and Jupiter's huge mass combine to wreak havoc on
Io. Tidal forces pull hard on the moon, causing its rocky
internal layers to rub against each other and melt from the
friction-induced heat. This process produces magma, which then
erupts in volcanoes. [ Infographic:
Inside Gas Giant Jupiter ]

Io is the only body in the solar system besides Earth known to
boast active lava volcanoes (some other satellites, such as
Saturn's moon Enceladus, harbor ice volcanoes). And Io is far
more volcanically active than Earth.

Previous theories had suggested that Io's molten rock may splash
about in a subsurface ocean extending across the entire moon. But
the new study is the first to offer hard evidence of that,
researchers said.

Khurana and his colleagues studied observations made by NASA's
Galileo spacecraft, which peered at Jupiter and some of its
satellites from 1995 to 2003. During four flybys of Io in 1999
and 2000, Galileo's magnetometer picked up a strange signal
coming from the moon.

The signal was an induced response to the rotating magnetic field
of Jupiter, and it was likely produced by an electrical current
in Io's subsurface rocks. That only makes sense, researchers say,
if the moon has a global layer of molten or partially molten rock
beneath its solid crust.

Rock of the type found on Io is not very conductive when it's in
solid form, but that changes dramatically when it melts. So the
signal is strong evidence for a widespread, interconnected magma
pool.

Other research teams have used similar data and methodology to
help deduce the presence of subsurface oceans on other
satellites, such as
Jupiter's moon Europa, Khurana said. But in those cases, the
conducting medium is thought to be liquid water, not liquid rock.

Further analysis and modeling suggested that at least 20 percent
of the rock in the reservoir must be melted, and that the magma
pool is 30 miles (48 km) deep at a minimum. It lies below Io's
crust, which itself is 20 to 30 miles (32 to 48 km) thick.

The magma ocean likely constitutes about 10 percent of Io's rocky
mantle by volume, researchers said. Khurana and his colleagues
publish their results tomorrow (May 13) in the journal Science.

Learning about Io, and about Earth

While the new study should help scientists better understand Io's
extreme volcanism, it could also shed light on other aspects of
the moon, researchers said.

For example, the results could help explain why, in contrast to
Earth, Io doesn't have a strong intrinsic magnetic field. Earth's
internal magnetic field is thought to be generated and maintained
by the circulation of convecting, electrically conductive fluid,
in a phenomenon known as the dynamo effect.

The new study could also provide researchers a sort of window
into the Earth's past, Khurana said. Both Earth and our moon are
thought to have possessed underground magma oceans like Io's, but
they cooled long ago.

That cooling paved the way for the establishment of
plate tectonics on Earth, which has shaped the planet we know
today. Studying Io could yield clues about how and when such a
huge transformation might have taken place.

"We all would like to know when plate tectonics began," Khurana
said.

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